A semiconductor structure includes a substrate, a buffer layer, a channel layer, a barrier layer, a doped compound semiconductor layer, and a composite blocking layer. The buffer layer is on the substrate. The channel layer is on the buffer layer. The barrier layer is on the channel layer. The doped compound semiconductor layer is on the barrier layer. The composite blocking layer is on the doped compound semiconductor layer, the composite blocking layer and the barrier layer include the same Group III element, and the atomic percent of the same Group III element in the composite blocking layer increases with the distance from the doped compound semiconductor layer.